1. Field of Invention
The present invention relates generally to a chemical mechanical polishing system. More specifically, the invention relates a method and apparatus for positioning a workpiece or semiconductor wafer into a chemical mechanical polishing system.
2. Background of Prior Art
In semiconductor wafer processing, the use of chemical mechanical polishing, or CMP, has gained favor due to the enhanced ability to stack multiple feature layers on a semiconductor workpiece, or wafer. As the demand for polishing wafers as part of a semiconductor fabrication process increases the requirement for higher processing rates while minimizing risk of wafer damage and contamination has correspondingly experienced greater urgency for improvement.
Two such CMP systems that address these issues are described in a commonly assigned patent to Perlov et al. (U.S. Pat. No. 5,804,507, issued Sep. 8, 1998) and in a commonly assigned patent to Tolles et al. (U.S. Pat. No. 5,738,574, issued Apr. 15, 1998). Both Perlov et al. and Tolles et al. are hereby incorporated by reference. Both Perlov et al. and Tolles et al. disclose a CMP system having a polishing apparatus that is supplied wafers from cassettes located in an adjacent liquid filled bath. A transfer mechanism, or robot, facilitates the transfer of the wafers from the bath to a transfer station. A pedestal within the transfer station rises to receive the wafer from the robot. The wafer is released from the robot and secured, or chucked, to the pedestal by a vacuum chuck. The pedestal is then retracted into the transfer station wherein the wafer is temporarily released from the pedestal so that a plurality of alignment jaws may manipulate the wafer into a position centered upon the pedestal. The wafer is then rechucked to the pedestal and the pedestal rises to engage and transfer the wafer to a retaining ring of a polishing head. The alignment of the wafer to the center of the pedestal ensures that the wafer will be properly fixtured into the retaining ring. After the wafer is fixtured into the retaining ring, a carousel moves the polishing head to a polishing station. After completion of the polishing process, the wafer is returned to the pedestal where a robot retrieves the wafer and moves it to the proper cassette located in the bath.
Although this process has proven to be an effective mechanism for performing chemical mechanical polishing, an improvement has been identified which could enhance the transfer of the wafer into the polishing head, reducing the risk of wafer damage and contamination, while reducing the dwell time of the wafer at the transfer station.
More specifically, the process of transferring the wafer into and out of a single load/unload pedestal with a transfer robot that can only transport one wafer at a time causes a bottleneck in wafer movement into and out of the polisher. Specifically, a single arm robot with a single end effector for gripping a wafer obtains a wafer from a wafer storage device, positions the wafer upon a single load/unload pedestal. The wafer enters the polisher and work is polished. The wafer is then returned to the load/unload pedestal for unloading and transporting to a wafer storage device. The single arm robot must then grip the processed wafer and transport it to a wafer storage device, place the wafer in the device, and index down and retrieve the next wafer to be processed in the polisher. The robot then transports the next wafer to the load/unload pedestal for drop off. Elimination of the dwell time or the dead time in which the load/unload pedestal is idle would improve the routing time required to process each wafer, and yield a corresponding increase in wafer throughput additionally, the use of wafer edge grip only would minimize physical contact with either the backside surface or the device surface of the wafer during transporting and wafer placement. This is of special concern since any abrasion, scratching or other damage could occur on either side of the wafer.
Therefore, there is a need in the art for an apparatus that facilitates wafer transfer into a CMP polishing head wherein the transfer time is reduced and the wafer is fixtured with a minimal risk of particulate contamination and wafer damage.
The disadvantages associated with the prior art are overcome by the present invention of a transfer station comprising at least one buffer station, a transfer robot, and a wafer loading assembly. Preferably, a plurality of buffer stations is used, e.g., an input buffer station and an output buffer station. In operation, an input/output robot places a semiconductor wafer into an input buffer station. The input buffer station supports the wafer on three pins proximate the edge of the wafer. The transfer robot has two gripper assemblies, each having pneumatic gripper fingers that grab the wafer. The fingers retain the wafer at three points on the edge of the wafer. The robot lifts the wafer from the input buffer station and rotates the gripper and wafer to position the wafer over the wafer load/unload assembly, then places the wafer down onto the load/unload assembly. The wafer load/unload assembly then loads the wafer into a polishing head of the chemical mechanical polisher. While the transfer robot is positioning the wafer into the wafer load/unload assembly, the input/output robot may be positioning another wafer upon the input buffer station. Once a wafer has completed the polishing process, the polishing head releases the wafer into the wafer load/unload assembly, and the transfer robot removes the wafer from the load/unload assembly. The polished wafer is then placed in the output buffer station by the transfer robot where it remains until the input/output robot removes the polished wafer from the transfer station. As the transfer robot has two gripper assemblies, the steps of releasing the wafer into the load/unload assembly and the output buffer can occur simultaneously.
More specifically, the transfer robot comprises a pair of opposing gripper assemblies that are pneumatically actuated. Each of the gripper assemblies is located on opposing distal ends of two transfer arms. At a point where the transfer arms meet forming preferably a right angle, the transfer arms are coupled to a rotary actuator and a vertical linear actuator such that the grippers can be rotated from the buffer stations to the load/unload assembly and the robot arms can be raised and lowered to facilitate loading and unloading wafers. The grippers themselves have three fingers, plastic cylinders with a notch cut into the cylinder, to grip the edge of the wafer therein. The grippers are biased in an open position by springs such that the distance between the fingers of each gripper is greater than the diameter of a wafer. When compressed air is forced into a pneumatic cylinder, the grippers move towards one another and a wafer is captured between the three fingers.
Each of the transfer arms has a gripper assembly located at a first end and terminates in a second end that is mounted to a vertical shaft. The shaft is coupled to a rotary actuator through a clutch such that the arm can be rotated about 350 degrees. The shaft is coupled to an interlock mechanism that ensures that the transfer robot is properly position over either a buffer station or the wafer load/unload assembly before the transfer robot is allowed to be lowered into the buffer stations or the loadcup to release or pick-up a wafer. The interlock also ensures that the transfer robot cannot rotate toward the wafer load/unload assembly when that assembly is loading (or unloading) a wafer into the polishing head. To allow the robot to access either of the buffer stations and the wafer load/unload assembly, the buffer station and the wafer load/unload assembly are aligned along a circular path having the shaft of the transfer robot at its center.
In another aspect of the invention, a buffer station for positioning a workpiece is provided. In one embodiment, the buffer station includes a central portion and a discontinuous circumferential lip extending therefrom. The central portion is adapted to support the workpiece. A perimeter of the central portion and the lip define a plurality of cut-outs disposed radially about the perimeter. The arrangement of the cut-outs and discontinuous lip allow for different types of robot grippers to access the workpiece supported by the buffer station.